In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of water, an organic material such as a monohydric alcohol, a polyhydric alcohol, or mixtures thereof.
An inkjet recording element typically comprises a support having on at least one surface thereof at least one ink-receiving layer. The ink-receiving layer is typically either a porous layer that imbibes the ink via capillary action or a polymer layer that swells to absorb the ink. Swellable hydrophilic polymer layers tend to take a relatively longer time to dry compared to porous ink-receiving layers.
Porous ink-receiving layers are usually composed of inorganic or organic particles bonded together by a binder. The amount of particles in this type of coating is often far above the critical particle volume concentration (CPVC), which results in high porosity in the coating. During the inkjet printing process, ink droplets are rapidly absorbed into the coating through capillary action, and the image is dry-to-touch right after it comes out of the printer.
Inkjet prints, prepared by printing onto inkjet recording elements, are subject to environmental degradation. They are especially vulnerable to damage resulting from contact with water and atmospheric gases such as ozone. Ozone bleaches inkjet dyes resulting in loss of density. The damage resulting from post-imaging contact with water can take the form of water spots resulting from deglossing of the top coat, dye smearing due to unwanted dye diffusion, and even gross dissolution of the image recording layer. To overcome these deficiencies, inkjet prints can be laminated. However, lamination is expensive and adds complexity to printing, since a film laminate typically requires a separate roll of material, typically a film laminate in which an adhesive layer is prepared via an additional coating step. If the laminate is of the transfer type there is also added waste in the form of the exhausted coated support from which the film laminate is transferred.
Accordingly, efforts have been made to provide, in the form of a single sheet, an image-recording medium that has a top fusible layer which functions as a latent protective layer. This layer is porous and generally comprises fusible thermoplastic particles. This latent protective layer is often characterized as an ink-transporting layer when it is not retentive of the ink or colorant, which passes through to an underlying layer. When the layer functions as an ink-transporting layer, fusing transforms it into a protective topcoat for the underlying image. This single-sheet media design thereby eliminates the need for lamination to protect inkjet prints.
Absent a binder for the particles, the particles in the porous fusible layer may be heat sintered during the drying step to afford a continuous layer. Sintered layers, however, are relatively fragile and easily damaged. EP 858,905A1, for example, relates to an inkjet recording element having a porous, outermost layer formed by heat sintering thermoplastic particles of latex such as polyurethane which layer may contain a slight amount of a hydrophilic binder such as poly(vinyl alcohol). However, there is a problem with this inkjet recording element in that it has poor resistance to mechanical abrasion when it does not contain a hydrophilic binder, and poor water-resistance when it does contain a hydrophilic binder.
In other words, the use of typical water-soluble binders to improve prefusing durability of the porous fusible layer is disadvantaged in that, after fusing, the protective layer becomes susceptible to damage by water. Hydrophobic film forming binders, therefore, are preferred to make the layer more robust. U.S. Pat. No. 6,723,397 B2 (Wexler), for example, relates to an inkjet recording element in which a support has thereon in order in the direction from the support: (a) at least one porous, ink-retaining layer, and (b) a fusible, porous ink-transporting layer of fusible, polymeric particles and a film-forming, hydrophobic binder. The film-forming, hydrophobic binder can be any film-forming hydrophobic polymer capable of being dispersed in water, preferably an acrylic polymer or a polyurethane.
Polymers exhibiting a “lower-critical-solution-temperature,” also referred to in the prior art as “thermosensitive polymers,” “thermoresponsive polymers,” “heat-responsive polymers,” or the like, have been used in inkjet recording elements for various reasons. Thermosensitive polymers have been used to decrease the drying time of aqueous coating compositions that comprise hydrophilic polymers, since a thermosensitive polymer can be miscible with hydrophilic binders below its lower-critical-solution-temperature, but become hydrophobic and hence less water retentive when its temperature rises above its lower-critical-solution-temperature. Thermosensitive polymers have also been used to provide a smoother or glossier surface in inkjet recording elements that are not fusible. Finally, thermosensitive polymers have been used as a porogen, for the purpose of creating pores in a coated layer.
For example, US Patent Publication No. 2004/0191433 A1 (Sakaguchi et al.) relates to a recording medium having a porous ink-receptive layer comprising inorganic fine particles (hence, not fusible) and poly(vinyl alcohol) (hence, hydrophilic) as a main component of a binder. The ink-receptive layer further comprises a polymer emulsion containing a thermosensitive polymer which shows a hydrophilic property below the “thermosensitive temperature” and a hydrophobic property above the thermosensitive temperature. The coating solution is preferably maintained at a temperature not lower than the thermosensitive temperature until it is applied as a coating. When the coating solution is applied to a substrate, it is immediately cooled to a temperature not higher than the thermosensitive temperature. The publication states that by using a poly(vinyl alcohol) as a main component of a binder, and using a thermosensitive polymer emulsion in combination, the coating solution strongly thickens when it is cooled to a temperature not higher than the thermosensitive polymer emulsion and a void structure can be maintained when it is dried at relatively potent drying conditions. An inkjet recording element having high glossiness and ink-absorption property with high productivity is thereby obtained.
Patent application publication JP 2004-216766 similarly describes a coating composition comprising a polymer compound that is water-soluble at temperatures below its lower critical solution temperature (LCST) and hydrophobic above its LCST used in combination with poly(vinyl alcohol).
Both of the aforementioned compositions comprising poly(vinyl alcohol) are unsuitable for a fusible protective layer for reasons already described.
US Patent Publication No. 2004/0115370 (Funakoshi et al.) discloses a coating composition for manufacturing an inkjet recording element that comprises a polymer emulsion containing a thermosensitive polymer. The coating composition further comprises organic or inorganic fine particles, preferably made from a metal oxide, preferably not larger than one micrometer. The manufacturing method for the inkjet recording element comprises coating the coating composition on a substrate at a temperature above the thermosensitive temperature (or point) and then cooling down to a temperature not higher than the thermosensitive point. Thus, Funakoshi et al. state that the coating liquid is preferably prepared and used at a temperature above the temperature sensitive point (paragraph 0101). Funakoshi et al. further state that the coating liquid has a relative low viscosity at temperatures above the thermosensitive point, but abruptly becomes thick (or forms a gel) when the coating liquid is cooled down to a temperature not higher than the thermosensitive point. This is said to produce a very smooth and homogeneous coating layer with a good surface state that can be retained even after a drying process. As shown in Table 1 of US Patent Publication No. 2004/0115370, a water-soluble polymer such as poly(vinyl alcohol) is optional in the coating composition. It is noted that the inkjet recording element disclosed in this patent is not fused, so its surface state and gloss are as coated.
US Patent Publication No. 2003/0165626 (Poncelet et al.) relates to a method for preparing a coated material comprising a hydrophilic-based binder, in which the binder is cross-linked with a temperature-sensitive polymer that is water-soluble at temperatures below its lower critical solution temperature (LCST) and hydrophobic above its LCST. Such a method is unsuitable for a porous, fusible layer because of the substantial presence of a hydrophilic binder in the layer.
International Patent Publication WO 2004/069548 (Vaughan et al.) relates to a composition comprising a temperature-sensitive polyacrylamide and hydrophilic polymer particles and a method of coating the composition at a temperature below the LCST and then warming the material to a temperature above the LCST to form voids around the particulates in order to increase ink absorption. The layer disclosed is not a fusible protective layer and is intended to absorb the colorant in the ink.
JP2001-180105 A to Seiko Epson discloses a method of making an inkjet recording element in which the coating composition comprises, in an example, a thermosensitive polymer, silica gel, and poly(vinyl alcohol). The method involves applying the coating composition at a temperature below the thermosensitive point and then heating the substrate to a temperature above the thermosensitive point. The presence of the thermosensitive polymer in its hydrophobic state emits moisture and increases the efficiency of drying.
In view of the above, thermosensitive polymers have been used in the prior art for a variety of reasons, in a variety of inkjet recording elements, under various conditions, but not in fusible protective topcoats in inkjet recording elements for the purpose of stain and water-resistance.